Channel coding is a very important component in wireless communication systems like UMTS, WLAN (Wireless Local Area Network) and WPAN (Wireless Personal Area Network). In packet based transmission systems like WLAN and WPAN, as well as mobile radio systems (UMTS, HSDPA), the latency of the used channel decoder is of major importance. The latency of a decoder, or the decoding latency, is the time between the reception of the last bit of a block to be decoded and the output of the first result bit, i.e., after decoding.
In mobile radio, the reduction of latency can simplify the implementation complexity and the packet operation of the systems. In WLAN or WPAN systems, latency is an important parameter which has to be taken into account to ensure the functionality of the higher layers (MAC layer and above) and the efficient use of the spectrum resource by reducing the idle times.
FIG. 1 illustrates a stop-and-wait protocol in IEEE 802.11 WLAN systems, which is an error free case. More precisely, a typical situation in an 802.11 type of protocol based on CSMA-CA multiple access method is depicted. The source TX is sending a data block of length N at position 1. The destination RX receives the data block after the transmission delay at position 2. After the end of the data block reception at position 3, the TX destination sends out an acknowledgement ACK at position 4. This acknowledgement signal needs to be received by the RX source after an SIFS interval (Short InterFrame Space) at position 5. In the 802.11a standard this SIFS interval is equal to 16 microseconds.
In a block based channel decoder, the decoding process can only start after the reception of the last bit in the block. Thus, the decoder needs to be very much overdimensioned in order to fulfill the timing requirements given by the standard. Thus, the channel coding of choice are convolutional coding schemes, which can already start the decoding with the first received bit. In this case, the available decoding time is much longer than in a block code case, especially when only small or no channel interleaving schemes are used.
Turbo-codes are other types of coding schemes which offer very good performance. However, due to the iterative structure of the decoder, the decoding latency is greater than the decoding latency of a convolutional code. Further, in case of turbo codes, larger blocks are better for the performance and the throughput, but lead to a higher latency.
For all these reasons, the latency problem of the turbo code decoder has generally prevented the use of this class of channel codes in the packet domain like the WLAN standard 802.11a/b/g. Consequently, in existing systems using packet transmission, either no coding or convolutional coders are generally used.
U.S. published patent application no. 2002/0194555A1 teaches that a reduction in the latency time can be obtained by reducing the size of the block at the cost of a correlative reduction in the performance of the system. European patent no. 1,337,063 teaches an analysis of a quality of service (QoS) for performing an equal segmentation of a frame to be turbo-code encoded to obtain an optimal length of the sub-frames.
However, as indicated above, such a length reduction is not compatible with good performance and throughput for turbo codes, but also more generally for blocks oriented codes, including also Low Density Parity Check codes (LDPC codes), for example.